fix(observers): preserve advert pages with missing origin keys

[n30nex]

What this PR does

Closes #106. A NULL packets.origin_pubkey makes ListObserverAdverts fail when sqlc scans the encoded value into a Go string, so one incomplete advert breaks the entire page.

Return explicitly typed, non-null text for that field. Unknown-origin adverts remain in the page with an empty nodePublicKey; known keys and ID-based pagination are preserved. The query and sqlc output are regenerated together. No schema or API type changes.

Type of change

• Bug fix
• Tests

Checklist

• go build ./... passes
• gofmt -l . is empty
• go vet ./... passes
• go test ./... passes
• SQL changes regenerated with sqlc 1.31.1
• I have read CONTRIBUTING.md

No new dependency or Swagger contract change.

Testing notes

The real HTTP/Store/PostgreSQL regression reproduces the reported cannot scan NULL into *string and HTTP 500 before the fix. It now returns both missing-origin and known-origin adverts and continues correctly through the numeric cursor. Fixtures use temporary tables and roll back.

Built and ran the full suite natively on the Pi 5 with PostgreSQL enabled, based on merged dev at e15f873. The merged migration-recovery PostgreSQL test also ran without skipping. The broader preview upgrade is validated separately from this one-query bug fix.

AI tools assisted implementation and validation under the contributor's standing approval for this effort.

The Pi preview now serves b3682b8 with current web e43de48f. An actual missing-origin advert loaded successfully, both feeds are advancing, and the paired activity/telemetry APIs and charts pass. Merged migrations were tested on a private 27,086-observation preview copy; those timings do not represent analyzer production scale.

[n30nex]

@MrAlders0n please review this change with your Claude agent as part of the September 13 Beacon review batch. The current candidate is b3682b8e5551; validation and Pi evidence are in the PR description.

Please check NULL-origin advert rows, known-key preservation and cursor behavior. This PR is linked to issue #106 for closure on merge.